The present invention relates in general to triggered voltage controlled oscillators and in particular to a triggered oscillator adapted for rapid retriggering.
A triggered oscillator circuit generates a periodic output signal when triggered by an input trigger signal so that the output signal is phase synchronized to the trigger signal with the start of the first cycle of the oscillator output signal beginning a fixed time after the start of the trigger signal. In a well-known triggered oscillator circuit, the output of a NAND gate is fed back to one input of the NAND gate through a delay circuit and a trigger signal is applied to another input of the NAND gate. While the trigger signal remains low, the output of the NAND gate remains high and the oscillator does not oscillate. When the trigger signal is driven high, the output of the NAND gate oscillates with a frequency determined by the delay of the delay circuit and continues to oscillate as long as the trigger signal remains high. The initial state change of the periodic NAND gate output signal follows the leading edge of the trigger signal only by the relatively short switching time of the NAND gate and therefore the oscillator output signal is substantially phase synchronized to the trigger signal.
In some applications it is necessary to resynchronize the oscillator output signal to the trigger signal relatively often, as for example when the oscillator is utilized to control the timing of waveform sampling in an equivalent time waveform sampling system, wherein a periodic waveform is sampled during successive sampling bursts and each burst is triggered by an event in the waveform. Ideally, a triggered oscillator should be able to resynchronize its output to a trigger signal in the event the trigger signal is deasserted and then reasserted in rapid succession. However, in the above-described triggered oscillator circuit a problem arises if the delay of the delay circuit is long compared to the time between deassertion and reassertion of the trigger signal. If the trigger signal is deasserted while the feedback input to the NAND gate is low and then reasserted before the feedback input goes high, the retrigger action will have no effect on the NAND gate output. Therefore in such a triggered oscillator of the prior art, the retriggering of the oscillator must be delayed after the trigger signal is deasserted by at least the "recovery time" required for the oscillator signal delay circuitry to return to steady state with the feedback signal held high. This recovery time is equal to the sum of the propagation delay of the NAND gate and the delay time of the delay circuit. What is needed and would be useful is a triggered oscillator with faster recovery time.